---
title: "Reply to mailbox inbox thread"
method: POST
path: "/mailboxes/{id}/inbox/threads/{thread_id}/reply"
tags: ["Mailboxes", "Messages"]
---

# Reply to mailbox inbox thread

`POST /mailboxes/{id}/inbox/threads/{thread_id}/reply`

## Request

- Base URL: `https://shipmail.to/api/v1`
- URL: `https://shipmail.to/api/v1/mailboxes/{id}/inbox/threads/{thread_id}/reply`
- Auth: one of:
  - HTTP bearer
  - OAuth 2 (authorization code, authorization URL `https://shipmail.to/api/auth/oauth2/authorize`, token URL `https://shipmail.to/api/auth/oauth2/token`)

Reply to the latest message in the inbound JMAP thread identified by thread_id within the mailbox identified by id. Both IDs are returned by the mailbox inbox endpoints.

## Headers

- `Idempotency-Key` string
- `X-Shipmail-Organization` string

## Request body

- ReplyToThreadRequest
  - `to` Recipient[], required: To recipients (at least one).
    - union: Email recipient. Either an object with address and optional name, or a string.
      - object
        - `address` string, email, required: Email address.
        - `name` string, nullable: Display name (optional).
      - string: Email address as a string. Accepts plain address ("user@example.com") or display name format ("Jane Doe <user@example.com>").
  - `cc` Recipient[]: CC recipients.
    - union: Email recipient. Either an object with address and optional name, or a string.
      - object
        - `address` string, email, required: Email address.
        - `name` string, nullable: Display name (optional).
      - string: Email address as a string. Accepts plain address ("user@example.com") or display name format ("Jane Doe <user@example.com>").
  - `html` string: HTML body. At least one of html or text is required.
  - `text` string: Plain text body. At least one of html or text is required.
  - `client_reference` string: Caller-defined correlation reference, unique within an organization and mode. Sending again from the same mailbox with a reference already in use returns the original message (201) and sends nothing; the new body is not compared. The same reference from a different mailbox returns 409.
  - `metadata` MessageMetadata: Up to 20 scalar values and 8 KB of JSON. Nested objects and arrays are rejected.
  - `source_rfc_message_id` string: Caller-selected source RFC Message-ID. Its domain must equal the actual sender domain.
  - `headers` OutboundHeader[]: Safe outbound headers. Shipmail, SES, routing, and identity headers are rejected. RFC 8058 requires List-Unsubscribe-Post: List-Unsubscribe=One-Click and an HTTPS List-Unsubscribe URL.
    - `name` string, required: List-Unsubscribe, List-Unsubscribe-Post, or a non-reserved X-* header.
    - `value` string, required
  - `scheduled_at` string, date-time: Optional UTC ISO 8601 time to send this reply later.
  - `track_reply` boolean: Track the conversation in the Waiting workflow. true tracks it, false explicitly untracks it, and omitting the field keeps the thread's current tracking: a reply sent on a tracked conversation stays tracked. A tracked conversation moves to Needs action when a human reply arrives; automated replies (out-of-office, auto-acknowledgements, delivery notifications) do not end the wait.
  - `sandbox_outcome` 'delivered' | 'bounced' | 'complained': Synthetic final outcome for a sandbox send. Only accepted with a test API key.

## Response `201`

Reply queued for delivery.

- Message
  - `object` 'message', required
  - `id` string, required
  - `mailbox_id` string, required
  - `thread_id` string, nullable, required: Deprecated and not stable: a legacy thread ID that changes when the mail server re-threads its IDs or a relayed send's placeholder key is replaced. It remains supported in thread paths; store conversation_id instead.
  - `conversation_id` string, nullable, required: Stable Shipmail conversation identifier (thd_...). Store this: it survives thread merges and mail-server re-threading, which thread_id does not.
  - `source_rfc_message_id` string, nullable, required: Source RFC Message-ID supplied by the caller or observed before relay.
  - `delivered_rfc_message_id` string, nullable, required: Exact provider-assigned delivered RFC Message-ID after Shipmail observes it in a recipient reply. Null before an observation and never reconstructed from the SES provider ID.
  - `client_reference` string, nullable, required: Caller-defined organization-scoped reference.
  - `metadata` MessageMetadata, required: Up to 20 scalar values and 8 KB of JSON. Nested objects and arrays are rejected.
  - `headers` OutboundHeader[], required: Safe outbound headers. Shipmail, SES, routing, and identity headers are rejected. RFC 8058 requires List-Unsubscribe-Post: List-Unsubscribe=One-Click and an HTTPS List-Unsubscribe URL.
    - `name` string, required: List-Unsubscribe, List-Unsubscribe-Post, or a non-reserved X-* header.
    - `value` string, required
  - `subject` string, nullable, required
  - `from_address` string, nullable, required
  - `to_addresses` Recipient[], nullable, required
    - union: Email recipient. Either an object with address and optional name, or a string.
      - object
        - `address` string, email, required: Email address.
        - `name` string, nullable: Display name (optional).
      - string: Email address as a string. Accepts plain address ("user@example.com") or display name format ("Jane Doe <user@example.com>").
  - `cc_addresses` Recipient[], nullable, required
    - union: Email recipient. Either an object with address and optional name, or a string.
      - object
        - `address` string, email, required: Email address.
        - `name` string, nullable: Display name (optional).
      - string: Email address as a string. Accepts plain address ("user@example.com") or display name format ("Jane Doe <user@example.com>").
  - `bcc_addresses` Recipient[], nullable, required
    - union: Email recipient. Either an object with address and optional name, or a string.
      - object
        - `address` string, email, required: Email address.
        - `name` string, nullable: Display name (optional).
      - string: Email address as a string. Accepts plain address ("user@example.com") or display name format ("Jane Doe <user@example.com>").
  - `attachments` AttachmentMetadata[], nullable, required: Attachment metadata (filename, size, content_type). Null if no attachments.
    - `filename` string, required
    - `size` integer, required: Size in bytes.
    - `content_type` string, required
  - `source` 'api' | 'dashboard' | 'newsletter' | 'inbound' | 'smtp' | 'assistant', required
  - `mode` 'live' | 'test', required: The API-key data plane that owns this resource. Test resources are isolated from live delivery, quota, suppression, and reputation.
  - `status` 'scheduled' | 'queued' | 'sent' | 'delivered' | 'bounced' | 'complained' | 'failed' | 'suppressed' | 'cancelled', required
  - `rule_disposition` object, nullable, required: Inbox rule evaluation recorded when this inbound tracked message was delivered. Null for outbound messages, messages ingested before rule disposition was recorded, or messages delivered while rule evaluation was unavailable.
    - `matched_rule_ids` string[], required: IDs of the inbox rules that matched this message, in evaluation order.
    - `stop_rule_id` string, nullable, required: ID of the matching inbox rule whose stop setting ended further rule processing, or null if processing was not stopped.
  - `scheduled_at` string, date-time, nullable, required: UTC ISO 8601 time when a scheduled message should be sent. Null for immediate messages.
  - `created_at` string, date-time, required
  - `updated_at` string, date-time, required

## Other responses

- `401`: Authentication failed.
- `403`: Insufficient permissions.
- `404`: Resource not found.
- `409`: Resource conflict.
- `422`: Validation failed.
- `429`: Rate limit exceeded.

---

[API](https://skmtc.dev/shipmail/apis/shipmail-api.md) · [All operations](https://skmtc.dev/shipmail/apis/shipmail-api/llms.txt) · [OpenAPI document](https://skmtc.dev/shipmail/apis/shipmail-api/revisions/2c23e59d7e30?raw)